The present invention relates to non-crystalline photovoltaic cells such as amorphous silicon or thin-film photovoltaic cells. In particular, the present invention relates to measures for improving the absorption of incoming photons within the photovoltaic absorbing layer.
Solar photovoltaic cells made of crystalline or multi-crystalline semiconductor material, such as silicon, are dominating the market. Such cells have a photon absorbing layer with a thickness of 200 to 400 μm, so that a substantial amount of high-quality silicon material is used for their fabrication. One approach to reduce costs for the production of photovoltaic cells provides solar cell structures that require less high-quality (crystalline) silicon material. However, due to the poor absorption properties of silicon, in particular at wavelengths lower than λ=800 nm, a reduced thickness of the absorbing layer also means a significant reduction in the amount of light absorption and hence a lower output power of such photovoltaic cells.
Reducing or avoiding the use of high-quality silicon material leads to absorbing layers made of amorphous silicon or thin-film material. These solar cell structures can have a substantially reduced thickness which improves the collection efficiency of electron-hole pairs, and, in case of amorphous silicon cells, reduced degradation effects.
In order to improve the efficiency, a number of different topologies for photovoltaic cells have been proposed. In document US 2010/0065102, a photovoltaic device has a structure made of a first surface on a first column and a second surface on a second column which are applied with a first and second light absorption media, respectively. The surfaces form a resonance cavity that can trap incident light to enhance light absorption.
From document US 2011/0197959 A1 a photovoltaic cell is known that has a thin-film semiconducting layer provided on a support substrate, wherein a plasmon resonance-generating metallic structure is provided on the semiconducting layer for resonantly coupling light into the absorbing layer and transporting photo-induced charge carriers out of the absorbing layer.
In document US 2009/0266413 A1, there is described a photovoltaic cell whereon on a light absorption layer an electrode is disposed which is configured with a grating that enables light incident on the grating to be scattered into the light absorption layer and traps incident light with particular polarizations and incident angles in the grating to interact with the light absorption layer.
Document US 2011/0030773 teaches that for thin-film photovoltaic cells some wavelengths are not absorbed well by the absorbing layer as much of the red and infrared light simply passes through. In particular, wavelengths of between 550 and 900 nm should be reflected back into the absorbing layers to increase the absorption and hence the conversion efficiency of the cell. Therefore, it is proposed to structure the metal substrate on which the absorbing layer is provided.
Document US 2011/0048519 discloses a photovoltaic device with an increased light trapping using a front side light trapping and a back side light trapping. A diffuser is provided to diffuse scattered photons that pass through the absorbing layer without being absorbed. Furthermore, it may be provided that an emitter layer on the backside of the photovoltaic cell is roughened or textured in order to increase the light trapping.
Document US 2009/0250110 A1 discloses a photovoltaic cell with forward scattering nanoparticles on its surface to forward scatter radiation that would otherwise be reflected away from the photoconversion material. Hence, the transmission of photons into the active semiconductor region of the photovoltaic device can be increased, wherein the increased transmission of photons results in a correspondingly increased optical absorption and photogeneration of electrical current. The range of wavelengths within which this effect occurs can be controlled via the structure and composition of the nanostructures.
Document S. H. Zaidi et al., “Diffraction Grating Structures in Solar Cells”, Photovoltaic Specialists' Conference, 2000, IEEE, August 2002, pages 395 to 398, discloses a texturing of c-Si photovoltaic films. By using a wide range of 1D and 2D grating structures, the effective path length can be enhanced since the generation profile with the grating is a combination of different modes traveling at different angles.
Document J. N. Munday et al., “Large Integrated Absorption Enhancement in Plasmonic Solar Cells by Combining Metallic Gratings and Antireflection Coatings”, Nano Letters, ACS Publications 2011, 11 (6), pages 2195 to 2201, Oct. 14, 2010, discloses that plasmonic gratings may lead to a large narrow-band absorption enhancement in photovoltaic cells.
Also in document Z. Yu et al., “Fundamental Limit of Light Trapping in Grating Structures”, Optics Express, Volume 18, Issue S3, pages A366 to A380, 2010, it is disclosed that light-trapping schemes can be used to enhance absorption in photovoltaic cells. Light trapping can be accomplished by introducing random roughness on the surface of the cell.
In general, there are numerous approaches to increase the path length for photons in the absorbing layer.